This work is devoted to a novel Co3O4/ZnO hetero-junction prepared via a facile sol–gel route and evaluated for solar-driven photocatalytic reduction of hexavalent chromium (Cr(VI)). Structural, surface analyses (XRD, XPS, SEM, BET, UV–Vis DRS) and photoelectrochemical (PEC) characterization confirm the formation of intimate p-n hetero- “Co3O4 / ZnO” nanostructures with enhanced visible-light absorption, and improved charge separation. The electronic structures of Co3O4 and ZnO were investigated using first-principles DFT + U simulations (CASTEP) combined with the optical spectroscopy. The calculated band gaps of Co3O4 (1.39 eV) and for ZnO (3.11 eV) show an excellent agreement with the experimental values (1.55 eV) and (3.15 eV) respectively, confirming the reliability of the DFT + U approach in describing the electronic behavior of both oxides. Under solar irradiation and at initial [Cr(VI)] = 30 mg L⁻1, the optimized Co3O4/ZnO junction {mass ratio 1:1, pH ~ 4, T ~ 25 °C, [C7H6O3] = 10–5 mol/L, and a dose of 1 mg/mL} achieved a complete Cr(VI) removal within 150 min., with an apparent rate constant (kapp) of 0.012 min−1 and a pseudo-first-order kinetic. Reusability tests over three cycles show minimal loss in activity (< 5%) and negligible Co leaching (≤ 1 mg L−1), indicating a good stability. We propose that the established heterojunction promotes rapid electron transfer from the light-excited Co3O4 conduction band to ZnO, suppresses recombination, and enhances selective electron availability for the Cr(VI) reduction. This study highlights Co3O4/ZnO as an efficient, robust photocatalyst for the elimination of Cr(VI) under solar illumination.
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